The present invention relates generally to digital data communications and more particularly to a method for tuning a spread spectrum receiver to the frequency of a digital signal which is within a specified frequency band.
Spread spectrum digital communications techniques are well known. Such spread spectrum digital communications techniques provide a means for facilitating low power radio communications, increased bandwidth usage, and in many instances provide a low cost means for complying with Federal Communications Commission (FCC) regulations.
One such spread spectrum technique is Direct Sequence Spread Spectrum (DSSS) which utilizes burst transmissions. This is a phase modulation technique wherein the transmission phase of the message changes rapidly, perhaps every few microseconds.
Generally a communication system is designed so that both transmitter and receiver operate at precisely the same frequency. However, in less expensive systems it may not be possible to fix the frequency accurately enough to permit reception. In this instance, it will be necessary for the receiver to quickly tune to the frequency of the transmitted signal, so as to facilitate the modulation of the message before the transmission burst finishes.
In view of the forgoing, it would be desirable to provide a method for tuning a spread spectrum receiver to the frequency of the desired digital signal which is known to be located within a specific frequency band.
The present invention specifically addresses and alleviates the above mentioned deficiencies associated with the prior art. More particularly, the present invention provides a method for tuning a spread spectrum receiver to a frequency of a digital signal within a specified frequency band. The method generally comprises the steps of coarse tuning the receiver while a preamble of the digital signal is being received, then fine tuning the receiver while the preamble of the digital signal is still being received, and then demodulating the signal after the frequency thereof has been determined via such coarse and fine tuning.
According to the preferred embodiment of the present invention, the step of coarse tuning the receiver comprises dividing the specified frequency band into a plurality of sub-bands, and sequentially measuring a received signal strength within each first sub-band until a received signal strength is greater than a predetermined threshold.
According to the preferred embodiment of the present invention, the step of fine tuning the receiver preferably comprises dividing at least one first sub-band which was found above to have a receive signal strength greater than a predetermined value into a plurality of second sub-bands when a receive signal strength is greater than the predetermined threshold. This process is repeated as necessary. Thus, when the first one of the first sub-bands having a receive signal greater than a predetermined threshold is not found to contain the frequency of the digital signal, then the next first sub-band having a receive signal strength greater than the predetermined threshold is divided into second sub-bands. In this manner, each first sub-band is divided into second sub-bands and checked for the frequency of the digital signal until the frequency of the digital signal is found, as discussed in detail below.
After a first sub-band has been divided into a plurality of second sub-bands, then the receive signal strength within each second sub-band is sequentially measured so as to determine which second sub-band contains the signal having a receive signal strength above a predetermined threshold.
Since, as those skilled in the art will appreciate, a plurality of the second sub-bands may have a receive signal strength greater than the predetermined threshold, according to the present invention that second sub-band which contains the greatest received signal strength is determined to contain the desired frequency of the digital signal. Thus, that second sub-band which has the greatest received signal strength is de-modulated.
As discussed above, two different methods are utilized in the selection of the desired first sub-band and the selection of the desired second sub-band. Selection of the desired first sub-band is performed by selecting the first sub-band having a receive signal strength greater than predetermined threshold and then the next first sub-band having a receive signal strength greater than the predetermined threshold is assumed to contain the frequency of the desired frequency of the digital signal. This process is continued until that first sub-band which actually contains the desired frequency of the digital signal is found.
By way of contrast, determination of the second sub-band (which is contained within one of the first sub-bands) is performed merely by selecting that second sub-band having the highest received signal strength. While this procedure avoids the interactive process used to determine which second sub-band contains the desired frequency of the digital signal, this simplified procedure can not be utilized efficiently to determine the desired first sub-band, since the greater band width of the set of first sub-bands make it much more likely that an undesirable signal will have a greater amplitude than the desired digital signal, thereby increasing the probability that one of the first sub-bands, other than the desired first sub-band, will contain the highest receive signal strength. That is, the increased band width of the set of first sub-bands makes it likely that selection of the first sub-band by utilizing the greatest signal contained within any of the first sub-bands will provide a false indication.
According to the preferred embodiment of the present invention the step of sequentially measuring a receive signal strength within each first sub-band comprises averaging a plurality of separate receive signal strength measurements within each sub-band.
According to the preferred embodiment of the present invention, the method for tuning a spread spectrum receiver to a frequency of a digital signal within a specified frequency band further comprises the step of verifying the second sub-band by demodulating at least a portion of the preamble of the digital signal.
The step of fine tuning the receiver preferably comprises fine tuning the receiver to within a frequency band which is small enough for an automatic frequency control circuit to pull the frequency of the digital signal within its control. Thus, the automatic frequency control circuit can then pull in and maintain the frequency of the digital signal.
The demodulization process preferably comprises demodulating a preamble of the signal in the second sub-band which has the greatest receive signal strength, demodulating frame synchronization data associated with that preamble, demodulating error detection coding associated with that preamble, demodulating a message length associated with that preamble, and finally, demodulating the message itself.
Thus, according to the present invention, a method for tuning a spread spectrum receiver to a frequency of a digital signal within a specified frequency band is provided.